You can find the impact factor of a journal by searching Journal Citation Reports, a database on the ISI Web of Knowledge Portal.
The impact factor is the average number of times articles from a journal published in the past two years have been cited in a JCR year. It is one metric that can be used to assess the importance of a journal in an individual discipline.
The Journal Citation Reports database allows you to search for a specific journal title, or view a list of journals by subject category, publisher, or country. It is divided into Science and Social Science editions.
For more information about measuring journal quality, and where to publish please refer to the Research Impact Subject Guide at http://libguides.library.usyd.edu.au/impact